Why are the Liberal Democrats offering to help Boris Johnson secure a pre-Christmas election?

It was on, then it was off, then it was on again. Now, Boris Johnson‘s bid to force an early general election looks set to succeed, thanks to the unlikeliest of allies.

The prime minister was handed a surprise boost at the weekend when the Liberal Democrats and SNP jointly announced that they were willing to support his calls for a pre-Christmas poll, despite the fact the two parties said the aim was to remove the Conservatives from power.

The two parties proposed a plan that would bypass Labour’s opposition to a December election, just as Jeremy Corbyn’s refusal to play ball had looked set to scupper Johnson’s ambitions.

While the Fixed-term Parliaments Act says that two-thirds of MPs must vote for an early election for a poll to take place – meaning trigger one is all but impossible without Labour’s support – the Liberal Democrats and SNP suggested a one-line amendment to the Act to allow an election on 9 December. That would only require the backing of a straight majority of MPs, and with the support of the second and third biggest opposition parties, would be certain to pass.

The move sparked fury from Labour, with John McDonnell, the shadow chancellor, accusing the Liberal Democrats of “selling out” the campaign for a second referendum.

His anger is easy to understand. Having fought for months to get Labour to back a Final Say vote, second referendum supporters like McDonnell did not expect the Liberal Democrats to suddenly, if temporarily, give up on the campaign and instead offer to support the Conservatives in triggering a snap election.

So what is Liberal Democrat leader Jo Swinson up to?

That depends who you ask. Lib Dems say their top priority is stopping Brexit, and that a general election is now the best way to achieve this.

For them, the risk of Johnson securing parliament’s support for his exit deal and delivering Brexit in the coming weeks is greater than the risk of him winning a majority at a general election. Having said they now accept that there is currently not a majority in parliament for a second referendum, they believe an election and the opportunity to defeat the Tories has become their best prospect of keeping the UK in the EU.

The Lib Dems also say that they have been warned by contacts in Brussels that the EU will not agree to a further Brexit delay beyond 31 January unless there is a good reason – for example, to allow time for a referendum. The believe this means that there is a still too high a chance of the UK crashing out of the bloc at the end of January. Ousting the Tories from power is the only way to take this off the table.

A less generous explanation is offered by their opponents. They have accused the Liberal Democrats of playing cynical party politics in the logical belief that they will perform better in a pre-Brexit election than in a poll after the UK has left the EU. As Independent Group for Change leader Anna Soubry put it: “I am sorry to say that old-style, selfish, tribal party politics is at play.”

In an election that effectively became a re-run of the 2016 Leave versus Remain referendum, the Liberal Democrats would be by far the biggest pro-EU party in England and Wales. They would be expected to capitalise on public frustration at Labour’s ambiguous Brexit position and win over large numbers of Remain voters who want to support an unashamedly anti-Brexit party.

A post-Brexit election, however, would likely be fought mostly on issues relating to public services and spending, which typically prove more fertile ground for Labour. With Brexit pushed down the agenda, the Liberal Democrats would be at risk of losing their main appeal for voters.

It is barely believable that this is not at least a factor in Swinson’s thinking. But for all the accusations that she is putting party politics ahead of the national interest, she would hardly be the first party leader to do so. Both Theresa May and Jeremy Corbyn have been guilty of that in recent months, and no one genuinely believes that Johnson would be pushing so hard for an election were the Tories not sitting on a commanding poll lead.

Still, campaigners for a second referendum will be furious at what they see as the Liberal Democrats’ betrayal, given the party has been among the most vocal in its support for a Final Say vote.

Swinson’s decision to temporarily accept defeat in demanding another public vote is, at best, a gamble. At worst it is reckless. If, as current polling suggests it will, the subsequent election gives Johnson a majority and allows him to deliver Brexit with ease early in the new year, Remain voters will not forget who gifted him the poll he craved.

The Liberal Democrats might emerge from the election with significantly more MPs than now, but their success could come at the cost of a significant proportion of their supporters feeling profoundly betrayed. It would, some may point out, not be the first time.